1. Field of the Invention
This invention relates to a method for controlling injection in a die casting machine and apparatus for controlling the same, more particularly, to an injecting control in a die casting machine capable of producing a high-quality die-casting product without fins.
2. Description of the Related Art
It is conventionally known that quality of a die-casting product is greatly influenced by an injection velocity or an injection pressure when molten material is fed into a die. Particularly, a sufficient pressurization is needed before the molten material is coagulated, therefore, a die casting machine having a two-stage driving cylinder device for injection and pressurization has been used.
Generally, in the aforementioned type die casting machine, after an injection plunger is advanced at low-speeds, the molten material is started feeding into a die cavity to avoid foaming the molten material and so on. The end of the molten material reaches a gate portion of the die, thereby the pressure of the injection cylinder device for feeding is increased. After that, the injection plunger is advanced at high-speeds to avoid decreasing the temperature of the molten material, and then the molten resin is rapidly fed into the die cavity.
Following an injection process as described above, at the time the pressure of the injection cylinder device is further increased by filling the die with the molten material or at the time the injection plunger is advanced to a predetermined position corresponding to completion of feeding, a high-pressure is added to the injection cylinder device by a pressurizing cylinder device to operate a pressurizing process for increasing the pressurization to the molten material fed into the die by the injection plunger.
(Concrete explanation of the conventional die casting machine of a pressurizing system)
The conventional die casting machine of the two-stage cylinder pressurizing system will be explained in detail below.
In the die casting machine 90 shown in FIG. 7, a molten material 92 fed into the die cavity 91 is charged into an injection sleeve 93, and injected by driving an injection plunger 94 by using an injection cylinder device 95 for feeding. And further, hydraulic fluid charged in a back side of the injection cylinder device 95 is charged with pressure at the high-pressure by using a pressurizing cylinder device 96 having a larger diameter after the feeding of molten material 92 is completed, the molten material 92 fed into the die cavity 91 is pressurized through the injection cylinder device 95.
FIG. 8 shows injection velocity change CV and injection pressure change CP in an injecting process to a pressurizing process in the die casting machine 90. In the drawing, the injection cylinder device 95 is advanced at a low-velocity VL at the outset, and, begins advanced at high-velocity VH at time t1 to feed the molten material 92 in a stroke, after that it is braked by receiving a feeding pressure of the molten material as feeding finishes. At time t2, the pressurizing cylinder 96 is driven to pressurize the molten material, thereby a pressure of the molten material 92 fed into the die cavity 91 reaches a pressure PH, and then the injection cylinder 95 is further advanced and stopped at time t3. This stopping point is a stroke end of the injection cylinder device 95.
For a linking control between the injection cylinder device 95 and the pressurizing cylinder device 96 in the die casting machine 90 (changing from the injecting process to the pressurizing process) as described above, a sequence valve system, in which the change is carried out with detection of an injection-pressure fluctuation, or a limit switch system, in which the change is carried out with detection of an advanced position of the injection plunger, is employed.
(Oil hydraulic circuit in the sequence valve system)
The following oil hydraulic circuit is used in the sequence valve system.
As shown in FIG. 9, the injection cylinder device 95 is connected with an injection oil hydraulic circuit 114 leading through a check valve 111 and an injection velocity adjusting valve 112 to an accumulator 113. The pressurizing cylinder device 96 is connected with a pressurizing oil hydraulic circuit 117 leading through a pressurizing controlling valve 116 of a pilot operation, opened by a sequence valve 115, to the accumulator 113.
The sequence valve 115 is defined to open the pressurizing controlling valve 116 when the pressure of the injection oil hydraulic circuit 114 exceeds a predetermined pressurizing-start pressure. Therefore, the injection cylinder device 95 is started advancing to inject by operating the injection velocity adjusting valve 112, and the feeding pressure is increased with the complete feed of the molten material into the die and reaches the predetermined pressurizing-start pressure, whereupon the sequence valve 115 is driven to open the pressurizing controlling valve 116, and then the pressurization is performed by starting advancing the pressurizing cylinder device 96.
FIG. 10 concretely shows a surrounding area of the injection cylinder device 95 and the pressurizing cylinder device 96.
The injection cylinder device 95 has an injection piston 95A therein, in which the injection piston 95A is advanced by an oil pressure of hydraulic fluid charged from the injection oil hydraulic circuit 114 to the back side of the injection piston 95A. The hydraulic fluid charged from the injection oil hydraulic circuit 114 flows to be adjusted by injection velocity adjusting valve 112, whereby the injection piston 95A is switched between advance and stop and an advancing velocity is adjusted.
The pressurizing cylinder device 96 has a pressurization piston 96A therein, in which the pressurization piston 96A is advanced by an oil pressure of hydraulic fluid charged from the pressurizing oil hydraulic circuit 117 to the back side of the pressurization piston 96A, thereby the injection piston 95A is pressed from the back side thereof through an intermediate member 95B of the injection cylinder device 95. The hydraulic fluid charged from the pressurizing oil hydraulic circuit 117 is intermittently controlled by the pressurizing controllable valve 116, whereby the pressurizing piston 96A is switched between advance and stop.
Intermittence of the pressurizing controllable valve 116 is carried out by the sequence valve 115. As the sequence valve 115, a solenoid valve, changing based on the feeding pressure by appropriate means, or the like is used.
Incidentally, in the conventional two-stage die casting machine as described above, the flow of the hydraulic fluid charged to the pressurizing cylinder device is fixed without an adjustable control in the pressurizing process in which the injection cylinder device is pressurized with the pressurizing cylinder device. Because, the hydraulic fluid charged to the pressurizing cylinder device is intermitted in the pressurizing controllable valve, in which a constant flow valve of an ON-OFF two-point switching type is conventionally used as the pressurizing controllable valve.
The hydraulic fluid is charged to the pressurizing cylinder device at a constant flow, so that a pressurizing properties of a casting pressure is to be the injection pressure change CP as shown in FIG. 8, in which the upturned curve makes a quadratic curve becoming gradually gentle slope as approaching the highest pressure PH. This is why the injection plunger receives resistance as the molten material in the die is being coagulated, so that the pressurization becomes slow. Concretely, the casting pressure P is proportional to the square root of the product of an elapsed time t and a coefficient a.
On the other hand, a fin critical pressurizing curve is known in the pressurizing process. The molten material leaks from a parting line of the dies when the exceeding pressure is added to the die in the pressurization, with the result that fins are produced. But, fins are not produced when the pressure is less than the fin critical pressurizing curve. The fin critical curve is given as a curve CX in which the casting pressure P is proportional to the product of the coefficient a and the square of the elapsed time t (see FIG. 8).
The reason why the fin critical curve CX is to be the quadratic curve as described above is because the molten material in the die, which does not coagulate in the early stage of the pressurization, easily leaks from the parting line in view of the high flow characteristics of the molten material, so that the high pressure cannot be added, but the molten material hardly leaks from the parting line after beginning to coagulate with the passage of time, so that fins are not produced although the high pressure is added.
In the aforementioned conventional die casting machine, however, the hydraulic fluid is charged to the pressurizing cylinder device in the constant flow, therefore, the pressurizing properties of the casting pressure is not allowed to approach the fin critical curve. As a result, the conventional die casting machine is presented with a disadvantage that the good quality die-casting products cannot be produced in view of substantially produced fins in the high-speed cast or the use of the die having the parting line with a low precision.
It is an object of the present invention to provide the method for injecting in the die casting machine and the apparatus for the same capable of producing the high-quality die-casting products, which are not produced with fins, even in a high-speed cast or the use of the die with a low precision.